We accept both spellings - I double-checked - but if you get something wrong we can't really affect what Duolingo will show you as the preferrable solution. Well, save for removing "humor" entirely as an accepted answer, which is obviously not good either. So it's just an automated system trying to help, not us actively ignoring British English.
Thanks, I appreciate it. :)
Having said the above, I'm certain we've missed non-American spellings occasionally - it would be surprising otherwise, given the size of the course. So if you get a correct answer that says you have a typo due to a British English spelling, please do report that!